Semiconductor devices such as Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are commonly used as power devices in applications, such as automotive electronics, power supplies, telecommunications, which applications require devices to operate at currents in the range of tenths up to hundreds of amperes (A).
Conventionally, by applying a voltage to the gate electrode of a MOSFET device, a channel will be formed connecting the source and the drain regions allowing a current to flow. Once the MOSFET device is turned on, the relation between the current and the voltage is nearly linear which means that the device behaves like a resistance. The resistance is referred to as the on-state resistance Rdson.
Typically, MOSFET devices with low on-state resistance Rdson are preferred as they have higher current capability. It is well known that the on-state resistance Rdson may be decreased by increasing the packing density of a MOSFET device i.e. the number of base cells per cm2. For example, a hexagonal MOSFET (HEXFET) device comprises a plurality of cells, each cell having a source region and a hexagonal polysilicon gate, and has a high packing density e.g. 105 hexagonal cells per cm2. Due to the large number of cells and the aspect ratio which may be defined as the ratio between the length of the hexagonal perimeter of the source region and the area of the unit cell, the on-state resistance of a HEXFET device can be made very low. Usually, the smaller the size of the cells, the higher is the packing density and thus, the smaller the on-state resistance. Therefore, many improvements to MOSFET devices are aimed at reducing the size of the cells.
However, as the size of the cells are reduced and the packing density increased, the breakdown voltage of the MOSFET devices are decreased. There is therefore a trade-off between reducing Rdson and having a high enough break down voltage BVdss.
As the cell size is reduced, the channel length is reduced until a limit is reached when the depletion layer width of the body region becomes comparable to the channel length causing punch-through current at high drain biases which impacts the break down voltage BVdss and causes degradation to the threshold voltage. In other words, as the channel length is reduced to a critical limit, short-channel effects arise which complicate device operation and degrade device performance, such as reduced threshold and breakdown voltage. There is therefore a limit below which the cell size cannot be reduced or improvements need to be made to eliminate or minimise the short-channel effects.
Also, as the breakdown voltage BVdss is lowered the unclamped inductive switching (UIS) capability is also lowered. UIS behaviour is associated with a parasitic bipolar transistor phenomenon which appears in the source body drain structure when the voltage on the drain is sufficiently high. A device with lower UIS capability has a larger resistance at the base of the parasitic bipolar transistor and an increased risk of transistor failure during momentary overloads. It is therefore common practice to measure the ruggedness of a MOSFET device by characterising its UIS behaviour.
There is therefore a need for an improved semiconductor device that has reduced Rdson while not degrading its breakdown voltage, threshold voltage or high energy capability (UIS).